The present invention relates to a clutch assembly. More particularly, the present invention relates to a more robust and readily manufacturable viscous fluid clutch (e.g., a magnetorheological (MRF) fluid clutch) for a fan drive assembly for use in a vehicle.
A viscous fluid clutch typically includes a viscous material, such as a magnetorheological fluid, operating in a gap between a driven rotor and a stator where the stator couples with the rotor to drive an output speed of the clutch and an attached fan blade assembly. Magnetorheological fluids typically include finely divided iron particles suspended in a non-polar medium. Magnetorheological fluids are preferably formulated to resist particle separation even under high separation force applications and typically function as Bingham fluids. In an ambient gravitational field and in the absence of a magnetic field, a Bingham fluid displays a shear stress that increases generally linearly as the shear rate on the fluid is increased. When a Bingham fluid is subjected to a magnetic field, the shear stress versus shear rate relationship is increased so that substantially more shear stress is required to commence shear of the fluid. Such a characteristic is useful in controlling transfer of torque between the rotor and the stator in an MRF clutch.
The known design of a viscous fluid clutch further includes a coil for creating an electromagnetic field in gaps between the rotor and the stator. When the magnetorheological fluid is subjected to the magnetic field, the yield stress of the magnetorheological fluid varies and the degree to which the stator is coupled to the rotor varies. In this manner, the output speed of the clutch is infinitely variable with respect to the input speed within the control range of the device.
In an engine driven fan system employing an MRF clutch, the speed of the fan is continuously variable by varying a magnetic flux density in the magnetorheological fluid. Such variable speed fan drive assemblies provide improved fuel economy, noise reduction, improved power train cooling, and cost reduction. However, conventional MRF clutches can involve excessive manufacturing cost and labor.
For example, in practice, all fan clutches, including conventional MRF clutches, have typically required the use of four or more fasteners to attach a fan blade hub to a fan cover body. The greater the number of fasteners, the greater the weight and cost of the final product and the more time required for manufacturing assembly.
Conventional MRF clutches also include a rotor having a slot, or a series of discontinuous slots (or other feature), to prevent the magnetic field from prematurely shunting across the rotor. The creation of the slots (or other shunt prevention feature) requires the rotor to undergo a complex additional machining process, which increases manufacturing cost and time.
Another disadvantage of conventional MRF clutches is that such clutches have proven to not be sufficiently robust for application in vehicles. For example, such clutches may include leak paths that enable the magnetorheological fluid to escape from the clutch as the MRF seeps into an internal porous portion of the cast aluminum fan cover body. Although the shell (or skin) of the casting generally prevents the fluid from leaking beyond the internal porous portion of the casting, bolt holes for attachment of the fan blade hub include machined threads. The machining process breaches the shell of the casting (which is created during the casting process) to expose the internal porous portion thereby providing a leak path for the escape of the magnetorheological fluid. Similarly, magnetorheological fluid can leak out of the clutch along a path formed by areas of contact between the cast fan cover body and a metal fan cover insert.
Additionally, in conventional MRF clutches, problems may arise during clutch operation. For example, in such clutches the clutch cover is typically positioned around a ferrous material cover insert. During operation of the clutch, the clutch cover and the clutch cover insert may tend to separate. Similarly, the rotor hub of such clutches may experience dimensional changes due to increased temperature during clutch operation. The dimensional changes can cause the rotor hub (and/or the rotor, which is attached to the rotor hub) to contact the clutch housing during operation.
MRF clutches typically generate a significant amount of heat due to viscous heating and are susceptible to damage from overheating. One disadvantage of conventional MRF fan clutches is that such clutches typically rely solely on incoming air flow (i.e., ram air) to cool the clutch. The ram air is generated by motion of the vehicle. When vehicle speed is low (e.g., at engine idle, during severe grade towing, travel with a significant tailwind), the velocity and volume of ram air flowing over the clutch may be insufficient to effectively cool the clutch. The velocity and volume of ram air reaching the fan clutch is also is also affected by restrictions to the free flow of incoming air, such as the vehicle front end, the radiator, the grille assembly, and the hood latch mechanism.
Additionally, conventional MRF clutches do not effectively direct the ram air to the cooling fins of the clutch. Due to clutch geometry, air flowing toward the clutch may stagnate or bypass the cooling fins so that heat is not effectively dissipated. For example, such clutches typically have an electrical cap connected to the fan clutch at a central area on the front of the clutch. The electrical cap creates a stagnation point so that heat cannot be effectively dissipated from the central area of the clutch. As a result, performance and overall durability of the clutch are reduced.
In a vehicle system, an MRF fan clutch is typically driven by the same pulley that drives the water pump. For example, a drive belt from the crankshaft pulley turns a water pump pulley, which drives both the water pump and the fan clutch. One disadvantage of such an arrangement is that the water pump and the fan clutch generally require different input speeds. Thus, the fan clutch must be stepped-up using an appropriate gear (pulley) device so that the input shaft of the fan clutch rotates at a proportionately higher speed than engine speed. Selection of the pulley ratio of the gear device requires a compromise between fan speed and water pump speed. If the ratio is too high, the fan speed may be excessive even though the water pump speed satisfies the demand for coolant flow. Excessive fan speed can cause premature failure of the fan clutch. Conversely, if the ratio is too low, the fan speed will provide insufficient airflow to the coolant flowing through the radiator resulting in diminished air conditioning performance at idle.